Method of and control system for controlling a nuclear reactor outlet temperature

ABSTRACT

The invention relates to a method of controlling an average temperature of a coolant at a reactor core outlet. The method includes the steps of detecting an actual average temperature of the coolant at the reactor core outlet, comparing the actual average temperature of the coolant at the reactor core outlet with a reference temperature thereby to generate an error signal, and adjusting the actual average temperature of the coolant at the reactor core outlet in response to the error signal. The invention extends to a nuclear reactor outlet temperature control system  10,  to a cascade controller for a nuclear reactor, and to a nuclear power plant.

[0001] THIS INVENTION relates to a method of controlling an averagetemperature of the coolant at a reactor core outlet. It also relates toa nuclear reactor outlet temperature control system and to a cascadecontroller for a nuclear reactor.

[0002] In a nuclear reactor plant, it is desirable that the reactor willnot become overheated. Accordingly, the rate of neutron generation andthe consequent rate of the fission reaction—the energy of which appearsas heat—must be controlled. This is achieved by positioning control rodsof a neutron absorbing material, which are insertable into the nuclearreactor core to a variable depth of insertion.

[0003] According to one aspect of the invention there is provided amethod of controlling an average temperature of a coolant at a reactorcore outlet, which method includes the steps of

[0004] detecting an actual average temperature of the coolant at thereactor core outlet;

[0005] comparing the actual average temperature of the coolant at thereactor core outlet with a reference temperature thereby to generate anerror signal; and

[0006] adjusting the actual average temperature of the coolant at thereactor core outlet in response to the error signal.

[0007] Adjusting the actual average temperature of the coolant mayinclude feeding the error signal, as well as signals corresponding to afluidic power and a neutronic power of the reactor to a control rodcontrol system, and controlling the position of the control rods inresponse thereto.

[0008] The method may include transforming the temperature error signalinto a power signal, combining the so-transformed power signal with ameasured reactor neutronic power and a measured reactor fluidic powerthereby to generate a control signal, and adjusting the actual averagetemperature of the coolant at the reactor core outlet in accordance withthe control signal.

[0009] Generating the control signal may include transforming a powererror signal, derived from the power signal, measured reactor neutronicpower and measured reactor fluidic power, into a control rod adjustmentsignal.

[0010] Adjusting the actual average temperature of the coolant at thereactor core outlet may include feeding the control rod adjustmentsignal into a control rod control system and adjusting a control rodinsertion depth in response thereto.

[0011] According to still another aspect of the invention there isprovided a nuclear reactor outlet temperature control system whichincludes

[0012] a detector for detecting an actual average temperature of thecoolant at a reactor core outlet;

[0013] a temperature comparator for comparing the actual averagetemperature of the coolant at the reactor core outlet as detected by thedetector with a set point temperature of the coolant at the reactor coreoutlet and for generating a temperature error signal;

[0014] temperature error signal transforming means for transforming thetemperature error signal into a power signal; and

[0015] control rod adjustment means for receiving the power signal andsignals corresponding to a reactor neutronic power and a reactor fluidicpower and adjusting the position of the control rods in responsethereto.

[0016] The control rod adjustment means may be in the form of a controlrod insertion depth controller for controlling the depth of insertion ofthe control rods into the nuclear reactor core.

[0017] The control rod adjustment means may include a power comparatorfor comparing a measured reactor neutronic power, a measured reactorfluidic power and the power signal, thereby to generate a power errorsignal. The control rod adjustment means may further include power errorsignal transforming means for transforming the power error signal into acontrol rod adjustment signal.

[0018] The control system may include reference means, coupled to thecomparator, for providing a manifestation of the set point temperature.

[0019] The control system may further include a reactor neutronic powersensor system, for sensing the reactor neutronic power and generating asignal of the measured value thereof, and a reactor fluidic power sensorsystem, for sensing the reactor fluidic power and generating a signal ofthe measured value thereof.

[0020] By reactor neutronic power is to be understood the rate ofneutron formation, and hence the rate of heat generation, in the reactorcore. The reactor neutronic power is therefore a variable derived fromneutron flux. By reactor fluidic power is to be understood the rate ofheat transfer to a working fluid of the reactor. Reactor fluidic poweris therefore a function of both the temperature gradient across thereactor core and the mass flow rate of the working fluid through thereactor core.

[0021] Adjusting the control rod insertion depth results in acorresponding change in the rate of neutron generation, and therefore inthe rate of the fission reaction and the reactor neutronic power. Thechange in neutronic power results in turn in a change in the averagetemperature of the coolant at the reactor core outlet.

[0022] More particularly, the invention consists of a cascade controllerfor a nuclear reactor, the controller having an inner loop and an outerloop, the inner loop regulating an error between a reactor neutronicpower and a reactor fluidic power by manipulating an insertion depth ofcontrol rods of the reactor and the outer loop regulating an averagetemperature of coolant at the reactor core outlet by manipulating anerror set point for the inner loop.

[0023] The invention will now be described, by way of example, withreference to the accompanying diagrammatic drawing, which shows aschematic diagram of a nuclear reactor outlet temperature control systemin accordance with the invention.

[0024] In the drawing, reference numeral 10 refers generally to anuclear reactor outlet temperature control system in accordance with theinvention.

[0025] The control system 10 includes a detector 16 for detecting anactual average temperature of the coolant at the reactor core outlet.The detector 16 is coupled to a temperature comparator 18. The system 10further includes reference means 17 coupled to the comparator 18, thereference means 17 providing a manifestation of a desired averagetemperature of the coolant at the reactor core outlet, commonly referredto as a set point temperature of the coolant at the reactor core outlet.

[0026] In use, the temperature comparator 18 compares an actual averagetemperature of the coolant at the reactor core outlet, as detected bythe detector 16, with a set point temperature of the coolant at thereactor core outlet, as manifested by the reference means 17, andgenerates a temperature error signal in accordance with the comparison.

[0027] The control system 10 includes temperature error signaltransforming means 20 for transforming the temperature error signalgenerated by the temperature comparator 18 into a power signal.

[0028] The control system 10 further includes a reactor neutronic powersensor 22, for sensing a reactor neutronic power, and a reactor fluidicpower sensor 24, for sensing a reactor fluidic power. The control system10 also includes a power comparator 26 to which the transforming means20 and each of the sensors 22, 24 are coupled.

[0029] In use, the power comparator 26 compares the neutronic power asdetected by sensor 22, the fluidic power as detected by sensor 24 andthe power signal from the transforming means 20, and generates a powererror signal in accordance with the comparison.

[0030] The control system 10 includes power error signal transformingmeans 40 for transforming the power error signal, generated by thecomparator 26, into a control rod adjustment signal. The control system10 includes control rod adjustment means 30, in the form of a controlrod insertion depth controller, which is configured to receive thecontrol rod adjustment signal transmitted from the transforming means 40and to adjust the depth of insertion of control rods of the nuclearreactor into the reactor core in response thereto.

[0031] The control system 10 includes two cascade control loops—an outercontrol loop or temperature control loop, generally indicated byreference numeral 12, and an inner control loop or power control loop,generally indicated by reference numeral 14—that is, an outer controlloop which operates an inner control loop in turn. The detector 16, thereference means 17, the comparator 18 and the transforming means 20 allform part of the outer control loop 12, the reactor neutronic powersensor 22, the reactor fluidic power sensor 24, the comparator 26 andthe transforming means 40 all forming part of the inner control loop 14.An output signal (that is, the power signal) of the outer control loop12 represents a function of the deviation of the actual averagetemperature of the coolant at the reactor core outlet from the set point(or desired) temperature of the coolant at the reactor core outlet. Thispower output signal triggers the inner control loop 14. The innercontrol loop 14 in turn controls the reactor neutronic power, viacontrol rod displacement, in accordance with the output power signal ofthe outer control loop 12.

[0032] The input signals for the outer control loop 12 are therefore theactual average temperature of the coolant at the reactor core outlet andthe set point temperature of the coolant at the reactor core outlet. Anerror of these two input signals is transformed into the power signal,which power signal constitutes the output signal of the outer controlloop 12 and is, in turn, an input signal for the inner control loop 14,together with the measured reactor neutronic power, as sensed by thereactor neutronic power sensor 22, and the measured reactor fluidicpower, as sensed by the reactor fluidic power sensor 24.

[0033] In use, the control system 10 is typically activated when thenuclear reactor is in a standby mode or in an operation mode, and duringtransitions between the different operation modes.

[0034] The invention extends to a nuclear power plant incorporating acontrol system in accordance with the invention.

[0035] In a nuclear power plant having a reactor unit and a powerconversion unit, the reactor unit facilitating the conversion of nuclearenergy into thermal energy which is transferred to the working fluid,and the power conversion unit facilitating the conversion of thermalenergy into electricity, the maximum temperature in a closed circuit forthe working fluid, which circuit interconnects the reactor unit andpower conversion unit, is set by the average temperature of the coolantat the reactor core outlet. The control system 10 in accordance with theinvention facilitates regulation of the maximum temperature in suchclosed circuit.

[0036] Furthermore, the inventors are aware of the problem of hunting ofreactor nuclear power (and hence of nuclear reactor core outlettemperature) which results in peaks (or spikes) in the nuclear powermagnitude, which peaks may be damaging to the nuclear fuel. Theinventors believe that by making use of the described integratedtemperature controller the problems of hunting and spikes will at leastbe alleviated.

1. A method of controlling an average temperature of a coolant at areactor core outlet, which method includes the steps of detecting anactual average temperature of the coolant at the reactor core outlet;comparing the actual average temperature of the coolant at the reactorcore outlet with a reference temperature thereby to generate an errorsignal; and adjusting the actual average temperature of the coolant atthe reactor core outlet in response to the error signal by feeding theerror signal, as well as signals corresponding to a fluidic power and aneutronic power of the reactor to a control rod control system, andcontrolling the position of the control rods in response thereto.
 2. Amethod as claimed in claim 1, which includes transforming thetemperature error signal into a power signal; combining theso-transformed power signal with a measured reactor neutronic power anda measured reactor fluidic power thereby to generate a control signal;and adjusting the actual average temperature of the coolant at thereactor core outlet in accordance with the control signal.
 3. A methodas claimed in claim 2, in which generating the control signal includestransforming a power error signal, derived from the power signal,measured reactor neutronic power and measured reactor fluidic power,into a control rod adjustment signal.
 4. A method as claimed in claim 3,in which adjusting the actual average temperature of the coolant at thereactor core outlet includes feeding the control rod adjustment signalinto a control rod control system and adjusting a control rod insertiondepth in response thereto.
 5. A nuclear reactor outlet temperaturecontrol system which includes a detector for detecting an actual averagetemperature of the coolant at a reactor core outlet; a temperaturecomparator for comparing the actual average temperature of the coolantat the reactor core outlet as detected by the detector with a set pointtemperature of the coolant at the reactor core outlet and for generatinga temperature error signal; temperature error signal transforming meansfor transforming the temperature error signal into a power signal; andcontrol rod adjustment means for receiving the power signal and signalscorresponding to a reactor neutronic power and a reactor fluidic powerand adjusting the position of the control rods in response thereto.
 6. Acontrol system as claimed in claim 5, in which the control rodadjustment means is in the form of a control rod insertion depthcontroller for controlling the depth of insertion of the control rodsinto the nuclear reactor core.
 7. A control system as claimed in claim 5or claim 6, in which the control rod adjustment means includes a powercomparator for comparing a measured reactor neutronic power, a measuredreactor fluidic power and the power signal, thereby to generate a powererror signal.
 8. A control system as claimed in claim 7, in which thecontrol rod adjustment means includes power error signal transformingmeans for transforming the power error signal into a control rodadjustment signal.
 9. A control system as claimed in any one of claims 5to 8, inclusive, which includes reference means, coupled to thecomparator, for providing a manifestation of the set point temperature.10. A control system as claimed in any one of claims 5 to 9, inclusive,which includes a reactor neutronic power sensor system, for sensing thereactor neutronic power and generating a signal corresponding to themeasured value thereof, and a reactor fluidic power sensor system, forsensing the reactor fluidic power and generating a signal correspondingto the measured value thereof.
 11. A cascade controller for a nuclearreactor, the controller having an inner loop and an outer loop, theinner loop regulating an error between a reactor neutronic power and areactor fluidic power by manipulating an insertion depth of control rodsof the reactor and the outer loop regulating an average temperature of acoolant at a reactor core outlet by manipulating an error set point forthe inner loop.
 12. A nuclear power plant which includes a nuclearreactor outlet temperature control system as claimed in any one ofclaims 5 to 10, inclusive.
 13. A method as claimed in claim 1,substantially as herein described and illustrated.
 14. A control systemas claimed in claim 5, substantially as herein described andillustrated.
 15. A cascade controller as claimed in claim 11,substantially as herein described and illustrated.
 16. A nuclear powerplant as claimed in claim 12, substantially as herein described andillustrated.
 17. A new method of controlling an average temperature of acoolant at a reactor core outlet, a new nuclear reactor outlettemperature control system, a new cascade controller for a nuclearreactor, or a new nuclear power plant, substantially as hereindescribed.